The present invention lies in general terms within the technical field of watermarking of digital images, more particularly fixed images.
The present invention concerns in particular a method of inserting a watermarking signal in a set of coefficients representing a digital image, in which at least one subset of coefficients is modified by said watermarking signal.
The invention also concerns a device able to implement such a method.
Watermarking digital data makes it possible to protect these data, for example by associating copyright information with them.
In its general principle, watermarking consists of inserting an indelible mark in digital data, similar to the encoding of additional information in the data.
The decoding of this additional information makes it possible to check the copyright information inserted.
This inserted watermark must consequently be imperceptible, robust to certain distortions applied to the digital image and capable of reliable detection.
Conventionally, the insertion of a watermarking signal in an image is obtained by modifying coefficients representing the image to be watermarked.
For example, a usual technique of inserting a watermarking signal in a digital image consists of using a linear modulation model in which at least one subset of spectral coefficients representing the digital image is modulated according to this linear model using a weighting coefficient.
Denoting X={Xi, 1xe2x89xa6ixe2x89xa6N} a set of coefficients representing a digital image and w={wj, 1xe2x89xa6jxe2x89xa6P} a watermarking signal of size P less than N, the linear insertion formula is:
X*j=Xj(i)+xcex1jwj 
in which Xj(i) designates a spectral coefficient amongst a subset chosen from the set X of spectral coefficients, and xcex1j is a weighting coefficient, also referred to as the modulation amplitude.
The detection of the watermarking signal then consists of detecting whether or not a modulation sequence has been inserted in a set of coefficients. This detection is carried out without using the original watermarked image and is based on a correlation calculation or on a standardised statistical test which makes it possible to theoretically calculate a probability of correct detection.
Such an insertion technique makes it possible, through the insertion of a watermarking signal, to insert a single information bit since the response of the detector is binary.
To insert a larger number of information bits in the digital image, in particular when a code of C bits indicating for example the name or address of the owner or author of the image is required, it is necessary to reiterate the insertion method described previously as many times as there are bits of information to be inserted.
Each weighting coefficient xcex1j must be greater than a minimum value, which can be termed the detection amplitude, so as to permit detection of the inserted signal with a correct level of probability.
This minimum value or detection amplitude depends notably on the length of the watermarking signal and on the level of detection probability required.
Moreover, each coefficient xcex1j must be less than a maximum value which can be termed the visual amplitude, denoted generally JND (Just Noticeable Difference) so as to meet the imperceptibility criterion of the watermarking signal.
It is known from the state of the art that the local content of the image to be watermarked has a visual masking effect which results in a drop in visual sensitivity making it possible to locally increase the modulation amplitude in the addition of the watermarking signal. Typically, luminance masking and contrast masking are spoken of.
It is then a case of applying psychovisual models in the domain of the image processing, and in particular in the compression of images.
The use of the results of psychovisual studies for adjusting the watermarking weighting in the domain of the watermarking of digital images is known from the state of the art.
For example, in the article entitled xe2x80x9cA DWTxe2x80x94based technique for spatio-frequency masking of digital signaturesxe2x80x9d by M. Barni, F. Bartolini, V. Cappellini, A. Lippi and A. Piva, which appeared in Proc. SPIE, pages 31-39, January 1999, a method of inserting a watermarking signal in the domain of the digital wavelet transformation (DWT) of a digital image is described.
This method uses a masking model previously proposed for image compression, in order to adapt the force of the watermarking to the characteristics of the human visual system. The model used makes it possible in particular to calculate the maximum weighting amplitude (xcex1j) allowed with respect to each representative coefficient (Xj) modified.
According to this model, this maximum weighting amplitude xcex1j is calculated from the energy of coefficients representing the neighbourhood of the representative coefficient Xj under consideration.
However, this method has the drawback of being of an analytical type, which entails many calculations for determining the parameters adapted to a given image. In addition, these parameters have to be recalculated for each image to be watermarked.
Moreover, it is known that the analytical masking models offer only an approximation of the masking effected by the human visual system.
The present invention aims to remedy the aforementioned drawbacks.
In this regard, the aim of the present invention is to propose a method of inserting a watermarking signal in an image by modifying coefficients representing the image, the modification of a coefficient under consideration taking into account the visual masking effect of the modification by the coefficients situated in a neighbourhood of the coefficient under consideration. The use of this method being very simple from the calculation point of view, and adaptive with respect to the type of images to be watermarked.
To this end, the present invention concerns a method of inserting a watermarking signal (w) in a set (X) of coefficients (Xi) representing a digital image (I), in which at least one subset of coefficients is modified by the watermarking signal (w). This method comprises, for each representative coefficient (Xi) to be modified, the following steps:
determining a neighbourhood (V(Xi)) of the representative coefficient (Xi) to be modified, in the image (I);
selecting a neighbourhood (Vsimd) in a dictionary of neighbourhoods representing coefficients representing the image (I), according to a predetermined criterion of similarity with the neighbourhood (V(Xi)) of the representative coefficient (Xi); and
modifying the representative coefficient (Xi) as a function of the watermarking signal (w) and a predetermined masking data item (M(Vsimd)) representing the masking effect on a watermarking signal of the neighbourhood (Vsimd) selected from the dictionary.
Taking into account a masking data item in the modification of a coefficient representing the image, it is possible to increase the degree of modification whilst meeting the invisibility criterion for the inserted signal. This makes it possible to obtain a better detectability of the watermarking signal in the watermarked image, or, at a fixed detectability level, to have a greater watermarking capacity, that is to say to be able to insert a larger number of watermarking bits. As this masking data item is obtained from a representative neighbourhood selected from a dictionary or stored table, the multiplication to be applied to the relevant coefficient of the image is calculated very rapidly.
According to a preferred embodiment of the invention, the method of inserting a watermarking signal comprises the following prior steps:
creating the said dictionary of neighbourhoods (Vd) representing the coefficients representing the image (I);
generating, for each neighbourhood (Vd) in the dictionary, a masking data item (M(Vd)) representing the masking effect of the neighbourhood on a watermarking signal.
Generating such a masking data item has the advantage of being able to use, when a watermarking signal is inserted, a psychovisual measurement of the modification made to the image, without using any analytical model. In addition, creating a dictionary of neighbourhoods allows an effective adaptation to the type of image processed.
According to a particular embodiment, the representative coefficients (Xi) are spatio-frequency coefficients obtained by a spatio-frequency transformation (T) of the image (I).
The representation of a spatio-frequency type is particularly adapted to an estimation of the masking effect since it corresponds to the representation of the human visual system, because of the separation of the image signal into two perceptual dimensions, frequency and orientation.
According to a preferred embodiment of the invention, the dictionary creation step comprises the following steps:
(A)xe2x80x94setting up a database of images, referred to as learning images, characteristic of a type of image to be watermarked;
(B)xe2x80x94for each learning image:
(b1)xe2x80x94selecting the coefficients representing the learning image under consideration, obtained by transformation (T), belonging to at least one sub-band (SB) of the image under consideration; and,
(b2)xe2x80x94for each coefficient (Xi) representing the sub-band (SB):
determining a neighbourhood (V(Xi)) of the representative coefficient (Xi) under consideration in the learning image under consideration;
calculating an energy (E[V(Xi)]) associated with the neighbourhood (V(Xi)) of the coefficient under consideration;
storing the neighbourhood (V(Xi)) in a database, referred to as the learning vector base, if the calculated energy (E[V(Xi)]) of the neighbourhood is greater than a predetermined threshold (E0);
(C) applying a predetermined processing to the vectors of the learning vector base so as to calculate a smaller number of vectors, the said calculated vectors constituting the dictionary of neighbourhoods.
By obtaining a dictionary comprising a reduced number of vectors, the quantity of information to be taken into account and therefore the calculation complexity is reduced. In addition, the use of a thresholding with respect to a calculated energy of a neighbourhood makes it possible to take into account only the neighbourhoods which have a real masking effect.
According to one characteristic of the above preferred embodiment, the dictionary of neighbourhoods is obtained by vector quantisation of the learning vector base.
Vector quantisation is an effective normal technique for reducing the quantity of information (here the number of vectors) to be considered, that is to say obtaining a reduced number of vectors representing a large number of vectors.
According to a preferred embodiment of the invention, the step of generating, for each neighbourhood (Vd) in the dictionary, a masking data item (M(Vd)) representing the masking effect of the neighbourhood on a watermarking signal, comprises the following steps, for each neighbourhood (Vd) of the dictionary:
applying the inverse of the transform (Txe2x88x921) to the neighbourhood (Vd) so as to obtain the corresponding image (Id);
modifying all the coefficients (Vid) of the neighbourhood (Vd) in successive stages, applying a modification value (M) varying according to an incrementation pitch (p) at each stage, and at each modification stage:
applying the inverse transform (Txe2x88x921) to the said modified neighbourhood (V*d) so as to obtain the corresponding modified image (Ixe2x80x2d);
effecting a perceptual measurement (MP(Id, I*d)) intended to evaluate a visual difference between the two images;
comparing the result of the perceptual measurement with a predetermined threshold (JND);
storing the modification value (M) when the result of the perceptual measurement reaches the predetermined threshold (JND), the modification value (M) stored constituting the masking data item (M(Vd)) associated with the neighbourhood (Vd) of the dictionary.
The use of a perceptual norm (JND threshold) avoids actual measurement by a human observer and therefore the subjectivity related to such a measurement.
According to one characteristic of the previous embodiment, the modification of each of the coefficients (Vid) of the neighbourhood (Vd) is carried out according to the following formula:
V*id=Vid+Mi 
in which V*id designates the modified coefficient and Mi designates an estimated modification value (M) for the coefficient (Vid) under consideration.
More particularly, in this embodiment, the modification of all the coefficients (Vid) of the neighbourhood (Vd) in successive stages is effected according to the following formula:
V*id=Vid+xcex1i(1+M) 
in which V*id designates the modified coefficient and xcex1i designates a weighting amplitude measured previously for the transformation (T) applied to a uniform image (that is to say with constant luminance, therefore creating no masking phenomenon), independently of the image, as a function of psychovisual criteria.
The use of this type of model expressed by the above formulae makes it possible to end up at the optimum case in terms of invisibility of the watermarking signal, independently of the image signal, when there is no masking effect. In addition, the model is controlled by a single parameter (M), which facilitates the use of a perceptual norm (JND threshold).
According to a particular embodiment of the invention, the step of modifying the representative coefficient (Xi) as a function of the watermarking signal (w) and a predetermined masking data item (M(Vsimd)) representing the masking effect, is effected by modulation in accordance with the following formula:
Xi*=Xi+Aiwi 
in which Xi* designates the modified representative coefficient Xi and in which Ai is a modulation amplitude calculated as a function of said masking data item (M(Vsimd)) representing the masking effect on a watermarking signal, of the neighbourhood (Vsimd) selected from the dictionary.
Thus the watermarking is effected at the limit of visibility taking into account the masking effect of the neighbourhood.
In the above embodiment, the modulation amplitudes Ai are used in the choice of a subset of representative coefficients to be modulated.
In this way, the modulation amplitudes Ai act in order to fix the detectability of the watermarking signal. This makes it possible in particular to increase the watermarking capacity (number of bits inserted) for a fixed degree of perceptibility.
According to a preferred embodiment of the invention, the transformation (T) applied to the image is a discrete wavelet transformation (DWT), and a neighbourhood of any representative coefficient Xi of the image is determined as being the oriented tree of wavelet coefficients which is in the neighbourhood of the representative coefficient Xi, the root of said tree consisting of the coefficient, referred to as the parent coefficient, which corresponds to the highest decomposition level in the tree.
In this way, the masking effects are taken into account according to the orientation, in accordance with psychovisual studies.
According to a particular implementation characteristic of the invention, the watermarking signal (w) is a predetermined pseudo-random sequence with a null mean.
This makes it possible to improve the detectability of the inserted watermarking signal.
According to a second aspect, the present invention concerns a device for inserting a watermarking signal (w) in a set (X) of coefficients (Xi) representing a digital image (I), in which at least one subset of coefficients is modified by the watermarking signal (w).
In accordance with the invention this device comprises notably, for each representative coefficient (Xi) to be modified:
means of determining a neighbourhood (V(Xi)) of the representative coefficient (Xi) to be modified, in the image (I);
means of selecting a neighbourhood (Vsimd) in a dictionary of neighbourhoods representing coefficients representing the image (I), according to a predetermined criterion of similarity with the neighbourhood (V(Xi)) of the representative coefficient (Xi); and
means of modifying the representative coefficient (Xi) as a function of the watermarking signal (w) and a predetermined masking data item (M(Vsimd)) representing the masking effect on a watermarking signal of the neighbourhood (Vsimd) selected from the dictionary.
This insertion device has characteristics and advantages similar to those described above since it is adapted to implement the insertion method according to the invention.
The present invention also concerns a computer, a digital image processing apparatus, a digital printer, a digital photographic apparatus, a digital camera and a scanner having means adapted to implement the insertion method according to the invention.
Correlatively, the present invention also concerns a computer, a digital image processing apparatus, a digital printer, a digital photographic apparatus, a digital camera and a scanner comprising an insertion device according to the invention.
These appliances have advantages similar to those described for the insertion method which they implement.
The invention also relates to a computer program containing one or more sequences of instructions able to implement the method of inserting a watermarking signal according to the invention when the program is loaded and executed in a computer.
The invention also relates to an information carrier, such as a diskette or compact disc (CD), characterised in that it contains such a computer program.
The advantages of this device, this computer, this computer program and this information carrier are identical to those of the method as succinctly disclosed above.
Other particularities and advantages of the invention will also emerge from the following description of preferred embodiments of the invention.